Electrochemical cells which are used for storing electric energy are generally referred to as battery or accumulator. Other electrochemical apparatuses are, for example, electrolysis cells. These can, for example, be used for preparing alkali metals from suitable salts comprising alkali metals.
Apart from batteries which are operated at ambient temperature, there are also ones which require an operating temperature above ambient temperature. These are generally electrochemical cells which operate using molten electrolytes, with the melting point of at least one electrolyte being above ambient temperature. Batteries of this type are, for example, those based on alkali metal and sulfur, with both sulfur and alkali metal being used in the molten state.
Batteries of this type which operate on the basis of a molten alkali metal as anode and a cathodic reaction participant, in general sulfur, are known, for example, from DE-A 26 35 900 or DE-A 26 10 222. Here, the molten alkali metal and the cathodic reaction participant are separated by a solid electrolyte which is permeable to cations. A reaction of the alkali metal with the cathodic reaction participant occurs at the cathode. This is, for example when using sodium as alkali metal and sulfur as cathodic reaction participant, the reaction of sodium and sulfur to form sodium polysulfide. To charge the battery, the sodium polysulfide is dissociated again into sodium and sulfur at the electrode by introduction of electric energy.
To increase the storage capacity of batteries based on a molten alkali metal and a cathodic reaction participant, batteries in which the amount of reactants used is increased by means of additional stock vessels are used. To effect discharge, the liquid sodium is supplied to the solid electrolyte. The liquid sodium serves simultaneously as anode and forms cations which are transported through the cation-conducting solid electrolyte to the cathode. At the cathode, the sulfur flowing onto the cathode is reduced to polysulfide, i.e. reacted with the sodium ions to form sodium polysulfide. The corresponding sodium polysulfide can be collected in a further vessel. As an alternative, it is also possible to collect the sodium polysulfide together with the sulfur in the vessel around the cathode space. Owing to the density difference, the sulfur rises and the sodium polysulfide settles. This density difference can also be utilized to bring about flow along the cathode. A corresponding battery design is described, for example, in WO 2011/161072.
DE-A 10 2011 110 843 discloses a battery based on molten sodium and molten sulfur, in which separate stores for sodium, sulfur and sodium polysulfide are provided and during operation the materials required in each case flow through the cells of the battery. The electrodes for taking off electric power are arranged on the sodium conduit and on the polysulfide conduit.
WO-A 2010/135283 describes a further sodium-sulfur battery in which separate vessels for sodium and sulfur are used in order to increase the range. Here, sodium and sulfur are each conveyed by means of pumps through the respective electrolyte spaces of an electrochemical cell. This results in continuous flow, so that the sodium polysulfide which is formed on the sulfur side is continuously discharged from the electrolyte space.
Since all electrochemical reactants are present in molten form and the optimal conductivity range of the ion-conducting ceramic membrane is attained only at relatively high temperatures, the operating temperature of such a battery is usually in a range from 300° C. to 370° C. The maximum temperature is generally determined by the degradation of the ceramic used as solid electrolyte.
To control the temperature of the cells, JP-A 2010-212099 or DE-A 40 29 901 disclose, for example, use of a temperature-control medium which flows around the electrochemical cells.
However, all these systems have the disadvantage that when a plurality of cells are joined to form a battery, no heat can be conducted away outward from the electrochemical cells located in the interior, as a result of which the cells located in the interior become significantly warmer than those located on the outside. This can frequently make a shutdown or throttling back of the power necessary during ongoing operation, which adversely affects the economics of the battery. In addition, heating and cooling of such a battery, for example for shutting it down or starting it up again, is possible with only very moderate temperature gradients over time and thus a large expenditure of time.
A further disadvantage of the batteries known from the prior art is that conveying apparatuses, in general pumps, which are in direct contact with the liquid sulfur or the liquid alkali metal have to be used for conveying the sulfur and the alkali metal. This leads to high corrosion rates and thus to frequent shutdown of the battery because of the necessary servicing and maintenance work.
It is therefore an object of the present invention to provide an apparatus for storing electric energy having at least one electrochemical cell and also a method of operating such an apparatus, which do not have the disadvantages known from the prior art.